In the graphic arts, it is desirable to produce a three or more color proof to assist a printer in correcting a set of color separations which will be used in exposing printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired half tone or line image. Visual examination of a color proof should reveal the color rendition to be expected from press using the color separations and any defects on the separations which might need to be altered before making the printing plates.
Color proofing sheets for multicolored printing can be made by using a printing press or proof press. This requires that all of the actual printing steps be formed. However, this conventional method of color proofing is costly and time consuming.
Alternate color proofing methods have therefore been developed to simulate the quality of press proofs. There are two general types of photoimaging methods, namely the overlay type and the single sheet type.
In the overlay type of color proofing method, an independent transparent plastic support is used for producing an image of each color separation film. A number of such supports carrying colored images are then superimposed upon each other and placed on a white sheet to produce a color proof. An example of this approach is described in U.S. Pat. No. 3,136,637. The primary advantage of this method is that it is quick and can serve as a progressive proof by combining any two or more colors in register. However, this type of color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet, and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by a conventional printing press proof.
In the single sheet type of color proofing method, a color proofing sheet is prepared by successively producing images of different colors from different color separation films onto a single receptor sheet. This is done by utilizing a single opaque support and by applying toners, photosensitive solutions, or coatings of photosensitive materials of corresponding colors on the opaque support in succession. An example of this approach is described in U.S. Pat. No. 3,671,236. An advantage of the single sheet type of color proof is that the color is not influenced by superimposed plastic supports. This method more closely resembles actual printing and eliminates the color distortion inherent in the overlay system.
Photosensitive layers can change their surface from tacky to non-tacky by incorporating photohardenable material. Colorants will adhere to the nonexposed, tacky areas and not to exposed, non-tacky areas. Such effects can be used to produce single sheet color proofing systems such as given in U.S. Pat. No. 3,649,268.
Phototackifiable diazonium salts can be used in photosensitive layers to change their surface from nontacky to tacky, as mentioned in the Journal of Imaging Science, 30 (1986), pp 86-89. These types of salts have been incorporated into photosensitive compositions, such as given in U.S. Pat. No. 4,334,006, for forming single images.